Commercial production of bioethanol and other useful products from lignocellulosic biomass requires high levels of feedstock throughput, on the order of 10 to 50 metric tons dry matter per hour. In biomass conversion systems that rely upon hydrothermal pretreatment of feedstocks prior to enzymatic hydrolysis, the scale of processing can be limited by the rate at which particulate material can be fed into pressurized pretreatment reactors.
Systems for “feeding” biomass into pressurized reactors generally fall into one of two predominant categories—plug feeders and sluice feeders. Plug feeders are well known in the pulp and paper industry. These use loading devices such as screws, pistons, and combination piston-screws to compact particulate material to a sufficient effective density so as to form a gas impenetrable pressure seal or “plug.” This plug is then continuously formed and loaded into a reactor against high pressures. Plug feeders have been reported to efficiently load against pressures from 4 to 10 bar. A variety of different plug feeder schemes have been reported. Some screw-plug feeders rely on a very long feeding screw, which permits loading at somewhat lower effective biomass plug density. Systems relying on higher effective density often utilize a disintegrator device on the pressurized reactor side to break apart the high density plug. See for example U.S. Pat. No. 3,841,465; U.S. Pat. No. 4,186,658; U.S. Pat. No. 4,274,786; U.S. Pat. No. 5,996,770; WO2003/050450; WO2004/105927; WO2009/005441.
Sluice feeders rely on a system of pressure locks, at least one of which is kept closed at all times. Particulate material is loaded into a sluice chamber through an open inlet valve. The inlet valve is then closed and the material unloaded into a high pressure reactor through an open outlet valve. A variety of sluice feeder systems have also been reported. See for example U.S. Pat. No. 5,095,825; SE 456,645; SE 500,516; WO1993/010893; WO1993/000282; WO2003/013714.
Individual sluice feeders generally have a lower capacity but provide a higher level of operational safety relative to plug feeders. Biomass is invariably heterogeneous material. Thus, even a highly compressed plug can contain channels through which potentially explosive release of pressurized steam may occur. In providing a mechanical valve seal against reactor pressure at all times, sluice feeders greatly reduce the risk of explosive release.
High density plug feeders have generally been considered advantageous over sluice feeders in that they can be readily scaled to very large capacity. However, plug feeders also have several notable disadvantages. Plug feeders have not been shown to operate effectively at pressures >10 bar. They are typically operated at very high levels of feedstock compression, in part to minimize occupational hazards. Biomass is typically pressurized to levels much higher than nominally required to seal against reactor pressure. As a consequence, plug feeders generate tremendous frictional forces between the plug and the feeding apparatus. This reduces energy efficiency and also introduces high levels of mechanical wear-and-tear, particularly with feedstocks having high sand or silica content such as wheat straw, rice straw and corn stover. Refurbishing of the “plug screw” or other loading device in plug feeders is routine maintenance which may be required on intervals as brief as 1-3 months. This introduces production inefficiencies as well as high maintenance costs. Plug feeders also typically require that feedstocks be subject to particle size reduction and extensive washing, which introduces additional process steps as well as increased energy requirements and running costs.
These disadvantages of plug feeders have been successfully avoided on a pilot production scale of 1 metric ton dry matter per hour using the single sluice chamber feeder system described in WO2003/013714, which is hereby incorporated by reference in entirety. Using this system, biomass can be efficiently loaded against pressures >15 bar. Feedstocks are processed without extensive particle size reduction or washing, first portioned into pre-determined portions, then force-loaded into a horizontal sluice chamber by means of a piston screw or similar device, the axis of which is practically in line with the axis of the sluice chamber.
We have discovered a variety of means whereby this sluice system can be scaled to larger capacity with increased operational safety and efficiency.
Further, we have discovered that sluice systems provide improved means for removing pretreated biomass from pressurized reactors. Plug feeders do not, themselves, provide means for removing pretreated biomass. Previously, pretreated biomass has typically been removed using “steam explosion” systems or “hydrocyclone” systems such as those described in WO 2009/147512, which is hereby incorporated by reference in entirety. Hydrocyclone systems were previously viewed as advantageous due to relatively conservative steam losses associated with removal of pretreated biomass. By using particle pump outlets to remove pretreated biomass, significant improvements over the performance of steam explosion and hydrocyclone systems can be obtained. In particular, relative to hydrocyclone systems, concentrations of the fermentation inhibitor furfural in the pretreated biomass released from the reactor can be reduced by more than 50%. Relative to steam explosion systems, both furfural content in released pretreated biomass and also steam losses associated with removal of pretreated biomass can be reduced.
These and other improvements are described in detail herein.